The invention relates to a method for converting Fischer-Tropsch type reaction products rich in C16-C50 linear paraffins into high purity C6-C24 normal alpha olefins (xe2x80x9cNAOsxe2x80x9d) having a purity of at least about 90 wt. %. This invention also relates to the conversion of C1-C3 alkane rich gases to more useful liquid hydrocarbons. In a further aspect it relates to the conversion of natural gas discharged in the recovery of crude oil, commonly referred to as flare gas, and excess C1-C3 alkanes produced as byproducts in various refinery operations, into more useful liquid hydrocarbon products such as normal alpha olefins, lubricating oil and liquid fuels. (The term liquid refers to hydrocarbons which are liquid at ambient conditions, including however, pentane.)
In the recovery of crude oil a large amount of natural gas (methane) is frequently encountered. In the past, depending on the location of the oil field, the value of the natural gas was frequently not considered to be worth the cost of recovery and transportation. Accordingly, in many cases, the natural gas which was generated was simply burned off. As well as being wasteful this practice is no longer considered acceptable from an environmental standpoint and in many cases prohibited by governmental regulations. A similar problem may also exist with respect to excess C1-C3 alkanes produced during petroleum refining operations or other chemical manufacturing operations to the extent it exceeds the fuel requirements of the facility. Thus, a need to convert natural gas or methane ethane and propane to more valuable products has been recognized for a number of years. Efforts have been undertaken since before World War II to convert methane to synthesis gas and synthesis gas (CO+H2) into more desirable liquid products and are still continuing today Typically these processes involve the use of the Fischer-Tropsch process, in which a less valuable material, e.g. coal or methane, is first converted to synthesis gas by incomplete oxidation and the synthesis gas converted to liquid or solid hydrocarbon products, e.g., paraffins, olefins and oxygenates. The Fischer-Tropsch products may in turn be upgraded to more useful products by a variety of operations. For example, U.S. Pat. Nos. 5,345,019 and 5,378,348 disclose a process for hydrocracking paraffins produced by a Fischer-Tropsch to produce kerosene, gas oil, and base oil. U.S. Pat. No. 4,943,672 discloses a process for producing lubricating oil from Fischer-Tropsch waxes by hydroisomerization. U.S. Pat. No. 4,579,986 is directed to a process for making C10-C20 linear olefins which comprises thermal cracking, in the presence of steam, C20+ paraffins obtained by a Fischer-Tropsch process using certain Fischer-Tropsch catalysts containing cobalt and zirconium, titanium and/or chromium. The patent also teaches that in addition to being useful as a feed for the preparation of linear C10-C20 olefins, the C20+ fraction is useful for obtaining solid paraffins, lower olefins (primarily ethene), high VI lubricating oil and middle distillates (Col. 4, lines 55-68) and that the C19-fraction may be used to prepare lower olefins, high VI synthetic lubricants, solvents and specialty oils (Col. 5, lines 1-23). U.S. Pat. No. 4,594,172 discloses a process for preparing high VI synthetic lubricants and U.S. Pat. No. 5,371,308 discloses a process for preparing lower olefins from a hydroprocessed synthetic oil fraction such as may be obtained from a Fischer-Tropsch synthesis. The general thermal cracking of petroleum waxes to produce normal alpha olefins is described in U.S. Pat. No. 4,042,488 and in The Oil and Gas Journal, pages 102-104, Dec. 13, 1965.
Many improvements have also been made in the basic Fischer-Tropsch process since its origins in the 1923, such that even though the Fischer-Tropsch process still produces a wide range of molecular weight products, the selectivity of the process may be directed between lighter paraffin and heavier paraffins (e.g. C20+ waxes) by adjusting reaction conditions and/or using different catalyst; see for example U.S. Pat. Nos. 4,041,097; 4,522,939; 4,579,986; and 5,378,348 and S. T. Sie, et al. Conversion of Natural Gas to Transportation Fuels via The Shell Middle Distillate Synthesis Process, Catalyst Today, Vol. 8 (1991) pp. 371-394 B. Jager, Developments in Fischer-Tropsch Technology, Studies in Surface Science and Catalysis, Vol. 107 (1997) pp. 219-224, and P. Chaumette, Gas to Liquid Conversionxe2x80x94Basic Features and Competitors, Petrole et Techniques, No. 415 (July-August 1998) pp. 83-85.
One of the problems with thermal cracking, at least where high purity normal alpha olefins are desired, is that the purity of the product is generally relatively poor because of the presence of dienes and branched olefins. Thus in the past ethylene oligomerization has bee used where high purity normal alpha olefins are desired. Therefore, it would be desirable to develop a process embodying thermal cracking which produces a high purity normal olefin product. Further, although much work has ben done with respect to Fischer-Tropsch processes and upgrading the products therefrom, it would be desirable to develop improved processes for converting Fischer-Tropsch reaction products into more valuable products especially in locations where the transportation costs associated with methane or other hydrocarbon gases are economically unattractive.
The present invention provides an efficient process for upgrading Fischer-Tropsch reaction products and for converting natural gas and other gases containing large amounts of methane ethane or propane or mixtures thereof into normal alpha olefins or other liquid hydrocarbon products particularly normal alpha olefins. The invention further provides a process embodying thermal cracking which produces a high purity C6-C24 normal alpha olefin product at least equal or better than that produced using the more expensive ethylene oligomerization processes. The C6-C24 normal alpha olefin products provided by the present invention contain at least 90 wt. % and preferably at least 95 wt. % C6-C24 normal alpha olefins. Further by using more rigorous separation processes purities of at least 98 wt. % approaching 100% can be obtained.
In one embodiment the present invention provides a process for upgrading Fischer-Tropsch products or product fractions comprising at least about 90 wt. % C16-C50 linear paraffins into high purity C6-C24 normal alpha olefin products which comprises the steps of:
a) thermal cracking the 90 wt. % C16-C50 linear paraffin mixture in the presence of steam at a mole ratio of steam to said mixture of at least about 5:1, under thermal cracking conditions adjusted to produce a cracking conversion of said mixture of about 30% or less thereby yielding a reaction product mixture comprising a fraction boiling within the C6-C24 normal alpha olefin boiling range, recomprising at least 90 wt. % C6-C24 normal alpha olefins.
b) fractionating the reaction product mixture of step a) into separate fractions comprising at least one normal alpha olefin product fraction comprising normal alpha olefins selected within the range of 6 to 24 carbon atoms in which said fraction has a normal alpha olefin purity of at least about 90 wt. % and a higher boiling fraction boiling above about 740xc2x0 F. (393xc2x0 C.) comprising higher boiling olefins and paraffins;
In another embodiment of the above process, full boiling range Fischer-Tropsch products are separated into a fuel fraction boiling below and about 540xc2x0 F. (282xc2x0 C.) a wax fraction boiling between about 540xc2x0 F. to 1100xc2x0 F. (593xc2x0 C.) containing at least about 90 wt. % linear paraffin and a high boiling fraction boiling from above about 1100xc2x0 F. (593xc2x0 C.). The wax fraction is thermal cracked as described above and one or more of the other fractions are hydrocracked to more valuable liquid hydrocarbon products. Similarly, the higher boiling fraction from step b) above may also be upgraded by hydrocracking.
The present invention also provides a process for converting C1-C3 alkane gases, e.g. natural gas, into more valuable products such as higher molecular weight liquid fuels and normal alpha olefins (NAO) which comprises the steps of:
a) reforming said C1-C3 alkanes into synthesis gas for example, by steam reforming, partial oxidation or catalytic oxidation;
b) contacting the synthesis gas with a Fischer-Tropsch catalyst under reactive conditions to yield two hydrocarbon product streams, one a wax containing product stream boiling above about 540xc2x0 F. (282xc2x0 C.) comprising C16-C50 linear paraffins, and a second product boiling below about 540xc2x0 F., comprising hydrocarbons boiling in the vacuum gas oil and liquid fuel ranges (e.g., paraffins, oxygenates and middle distillate, gasoline) and tail gases;
c) distilling the wax containing product of step b) into fractions comprising a linear C16-C50 paraffin fraction boiling in about the range of 540xc2x0 F. (282xc2x0 C.) to 1100xc2x0 F. (593xc2x0 C.) containing at least about 90 wt. % linear C16-C50 paraffins, a liquid fuel fraction boiling below about 540xc2x0 F. (282xc2x0 C.) and a heavy fraction boiling above about 1100xc2x0 F. (593xc2x0 C.);
d) thermal cracking the linear C16-C50 paraffin fraction of step c) in the presence of steam at a steam to said C16-C50 paraffin fraction mole ratio of at least about 5:1 under thermal cracking conditions adjusted to produce a conversion no greater than about 30 wt. % to produce a reaction product mixture comprising a substantial amount of C6-C24 NAOs without the formation of significant amounts of dienes;
e) fractionating the reaction product of step d) into NAO product fractions of varying chain length within the range of C6-C24 having a NAO content of at least 90 wt. % and a higher boiling fraction boiling above about 1100xc2x0 F. (593xc2x0 C.) containing branched olefin, paraffins and NAO""s having more than 24 carbon atoms;
f) hydrocracking the liquid fuel portion of the second product of step b); the vacuum gas oil fraction of step c) and the higher boiling fraction recovered in step e) with hydrogen in a hydrocracker in the presence of a hydrocracking catalyst under hydrocracking conditions to produce a mixture comprising gasoline and middle distillate; and
g) fractionating the reaction product of step f) and recovering at least one liquid fuel fraction, and at least one higher boiling hydrocarbon fraction and recycling at least one of said higher boiling hydrocarbon fractions back to said hydrocracker.
In another embodiment the invention provides a process for upgrading a substantially full boiling range Fischer-Tropsch reaction product including tail gases through bright stock boiling range hydrocarbons, which process comprises the steps of:
a) fractionating said Fischer-Tropsch reaction product into separate fractions comprising a fraction boiling in the liquid fuel boiling range, a wax fraction boiling in about the range of about 540xc2x0 F. to 1100xc2x0 F. comprising at least 90 wt. % C16 to C50 linear paraffins and a high boiling fraction boiling above about 1100xc2x0 F.;
b) thermal cracking the wax fraction of step a) in the presence of steam at a mole ratio of steam to said wax fraction of at least 5:1, under reactive conditions adjusted to produce a conversion based on said wax fraction no greater than 30 wt. % to yield a reaction product mixture containing a substantial amount of C6-C24 normal alpha olefins without the fomation of significant amounts of C6 to C24 dienes;
c) fractionating the reaction product of step b) into separate fractions comprising at least one normal alpha olefin product fraction comprising a normal alpha olefin fraction selected within the range of 6 to 24 carbon atoms having a C6-C24 normal alpha olefin purity of at least 90 wt. % and a higher boiling fraction comprising higher boiling olefins and paraffins;
d) hydrocracking said higher boiling fraction of step c), and the liquid fuel fraction of step a) with hydrogen in a hydrocracker in the presence of a catalyst comprising a hydrogenation component and an acid catalyst cracking component, under hydrocracking conditions to produce a liquid reaction product mixture comprising liquid fuel boiling hydrocarbons; and
e) fractionating the liquid reaction product mixture of step d) into separate fractions comprising a liquid fuel fraction, and at least one higher boiling hydrocarbon fraction and recycling at least one of said higher boiling fraction back to said hydrocracker.
In another embodiment the invention provides a process comprising the steps of:
a) converting C1-C3 alkanes into synthesis gas for example, by steam reforming, partial oxidation or catalytic oxidation;
b) contacting the synthesis gas with a Fischer-Tropsch catalyst under reactive conditions to yield a reaction product mixture of hydrocarbons comprising linear C16-C50 paraffins, vacuum gas oil, middle distillate, gasoline light oxygenates and light olefins;
c) fractionating the Fischer-Tropsch reaction product mixture of step b) into separate fractions comprising a linear C16-C50 paraffin fraction containing at least about 90 wt. % linear C16-C50 paraffin, at least one liquid fuel fraction and at least one higher boiling fraction boiling above the temperature of the C16-C50 rich fraction;
d) thermal cracking the linear C16-C50 paraffin fraction of step c) in the presence of steam at a mole ratio of steam under reactive conditions adjusted to produce a conversion based on said linear C16-C50 paraffin fraction of about 30 wt. % producing a mixture of NAO""s of varying chain length as a substantial product without the formation of significant amounts C6-C24 dienes;
e) fractionating the reaction product of step d) into NAO product fractions of varying chain length within the range of six to twenty-four carbon atoms having an NAO purity of at least 90 wt. % and a higher boiling fraction containing NAO""s having more than 24 carbon atoms and branched olefins and paraffins;
f) hydrocracking at least one of the liquid fuel fraction and higher boiling fractions recovered in step c) and the higher boiling fraction recovered in step e) with hydrogen in the presence of a hydrocracking catalyst under hydrocracking conditions to produce a reaction product comprising liquid fuel hydrocarbons; and
g) fractionating the reaction product of step f) and recovering at least one liquid fuel fraction and at least one higher boiling hydrocarbon fraction and recycling at least one higher boiling hydrocarbon fraction back to said hydrocracker.
Additional aspects of the invention will be apparent from the description which follows: